Pipes are often used to carry fluids or abrasive slurries as part of a treatment or disposal process. As the fluid passes through the pipe, frictional forces against the pipe walls can cause the pipe walls to wear. The effect of these frictional to forces is magnified on the curved portions of the pipe as a result of complex flow patterns and the forces from the fluid impacting the pipe surface caused by the fluid changing direction. Abrasives suspended within the fluid (e.g. solids commonly associated with mining slurries or sludge) can be even more detrimental to the pipe walls. Whatever the cause, these forces cause the pipe walls to wear even faster, is eventually causing the pipe walls to crack or burst where the walls have worn thin.
To minimize the effects of these frictional forces on the pipe walls, a protective coating is applied (e.g., welded or sprayed) to the inner surface of the pipe. To provide effective protection against wear on the pipe wall, it is important that the entire area is completely coated with the coating material. Areas left unprotected will continue to be vulnerable to fracture.
A prior patent owned by the present Applicants which is U.S. Pat. No. 6,171,389 which corresponds to Canadian Patent Application 2,302,083 (Hannu) published Oct. 12th 2000 discloses an apparatus for coating an interior surface of a pipe which has a support rod with a substantially square cross section that extends axially along the interior surface of the pipe. An index carriage slidably engages the substantially square cross section of the support rod so that the index carriage maintains a fixed rotational orientation relative to the support rod. An index motor having a variable index position is carried by the index carriage. An electric welding head for coating the interior surface of the pipe with weld material is rotatably attached to the index carriage and connected to the index motor. As such the head has an elevation position that corresponds to the index position of the index motor. A drive motor carried by a drive carriage propels the drive carriage longitudinally along the support rod. The drive carriage and the index carriage form a train with a series of intermediate carriages that slidably engage the substantially square cross section of the support rod to move longitudinally along the support rod
The welding torch applies an abrasion resistant surfacing such as tungsten carbide to the interior wear surfaces of large industrial pipe elbows and S-bends. The abrasion resistant surface is essentially formed through a welding process where layer after layer of weld beads are applied to the pipe in the axial direction.
A machine has been built and used extensively based on this patent and has achieved considerable commercial success. It does however have a number of areas where problems can arise and where improvements are desirable. On this machine, the tracking bar is unsupported over the free length that the welding torch travels. This limits the travel distance because as the unsupported length of the bar is increased oscillation and deflection problems are encountered. Presently the maximum travel distance is 11 feet.
Instability in the support system can create two main obstacles to fast, efficient welding of the pipe elbows and S-bends. First is the droop of the guidance bar in the center of the pipe. This droop hinders the ability of the welder to complete a straight and even path through the pipe. Furthermore, the larger the bend radius of the pipe, the greater the effect of droop has on the quality of the welds.
A second area of concern is with respect to the unwanted oscillatory motion that can develop in the guidance bar. Clearly this affects the ability to produce a straight, even and acceptable weld.
The current welding set-up is in some cases not stable enough to reliably produce fast and accurate welds in both directions of travel. Instability can develop into an unintended oscillating motion of the welding head that is entirely incompatible with the welding process used to layer the pipe. Due to the instability of the guiding system the operators generally can weld in one direction only. This combined with difficulty to rapidly return to the starting weld position can dramatically affect the welding cycle time.
Within a welding environment there come a number of challenges. First is temperature, expected to range between 250 and 450 degrees Fahrenheit. In addition to temperature, welding smoke, ultraviolet light and weld splatter must all be taken into consideration.
Another prior example of the prior art in this field is disclosed in U.S. Pat. No. 4,513,443 (Kostecki). Kostecki discloses an apparatus for coating an internal wall of a curved pipe with a layer of protective material. The apparatus has a guide means, a longitudinally flexible member, means for applying a coating, means for incremental rotation by the longitudinally flexible member, a drive means to drive the longitudinally flexible member relative to the curved pipe, and a means to automatically step the flexible member and consequently rotate the coating means. However, a disadvantage of using the flexible member in Kostecki is the associated inaccuracy in translating the rotational steps along the longitudinally flexible member. Because the coating means cannot be accurately rotated, the protective material can be misapplied resulting in either gaps or overlap between the applied strips of protective material.